[media] ivtv: prepare ivtv for adding const to s_register

The ivtv_itvc function receives a pointer to v4l2_dbg_register. When we
add const to that pointer in the s_register case we will run into a problem
here since this common function would discard const in that case. So
change this function so it receives the address and a pointer to a value.
In addition we now set the size field in the g_register function which is
where it belongs.
This will simplify the next patch where const will be added to s_register.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>